A beam splitter contains: a body, a main reflection portion, a sub reflection portion, and a refraction portion. The body includes an inlet and an outlet. The main reflection portion is located on a first side of the body, and the main reflection portion and the outlet have a first rotating angle and a second rotating angle respectively so that the main reflection portion reflects an external beam to produce a main beam. The sub reflection portion is located on the first side of the body, and the sub reflection portion reflects the external beam to produce a sub beam. The refraction portion is located on a second side of the body and has a third rotating angle different from the inlet, the sub beam projects out of the refraction portion to produce a deflective projection angle of the sub beam.